FreeBSD7+KDE3, Ctrl-Alt-F9 : no signal

[Pieter Donche]

Hi,

FreeBSD7-amd + X + KDE3 installed.

When connecting from a WindowsXP PC via an X-windows emulator, everything
is OK (1280x1024 screen)

On a console monitor (capable of 1280x1024) plugged in directly in
VGA connector of the machine (and a mouse and keyboard also),
Ctrl-Alt-F1 gives Command line interface (and reponds to keystrokes),
but when switching via Ctrl-Alt-F9 for X, I get a black screen,
and after 2 seconds the LED on the monitor turns from green to orange 
(meaning: no signal)

What can be wrong and how to remedy?